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Object Statistics

Also Extensions Created Because of SQL Plan Directives Can Invalidate Packages – Take Two

In a previous post I described a situation where the creation of an extension, independently of whether it’s carried out manually by a person or automatically by the database engine, can lead to the invalidation of PL/SQL objects. In this second post on that subject, I describe, with the help of an example (I love examples!) based on the extension_invalidate_pkg_remote.sql script, what can happen when the table on which the extension is created is stored in a remote database.

Let’s start by creating the required objects. Notice that all objects are local but, because the package body references the table through a database link that points to the very same schema owning all other objects, the database engine considers it a remote table.

Also Extensions Created Because of SQL Plan Directives Can Invalidate Packages

UPDATE 2015-12-18: As metioned by Bryn Llewellyn in this tweet, the invalidation I describe below is caused by bug 19450314. A patch for it is available here for several releases (e.g. it is available for, or, but not for I successfully tested it on

The fact that an extension explicitly created by a user through DBMS_STATS can invalidate objects like packages is not new in 12c. It has been like that since the introduction of extensions in 11g. In my opinion, since such an invalidation takes place only when a developer or DBA triggers it, I do not consider it a major problem.

Restrict the query optimizer through OPTIMIZER_FEATURES_ENABLE and Top-Frequency/Hybrid Histograms

To make upgrades easier, I regularly see people considering disabling query optimizer features by setting the OPTIMIZER_FEATURES_ENABLE initialization parameter to a non-default value. My general opinion about this “habit” is summarized in TOP with the following two sentences:

Changing the default value of the OPTIMIZER_FEATURES_ENABLE initialization parameter is only a short-term workaround. Sooner or later the application should be adapted (optimized) for the new database version.

Extension Bypassed Because of Missing Histogram

Today, while tuning a fairly complex query experiencing wrong cardinality estimates, I noticed something I was not aware of. Hence, I thought to write this short post to illustrate how to reproduce the problem I experienced…

Challenges and Chances of the 11g Query Optimizer

Challenges and Chances of the 11g Query Optimizer is the name of a presentation I gave at several events (e.g. Trivadis Performance Days, Oracle OpenWorld, DOAG Konferenz, UKOUG Conference) throughout 2011. Its abstract is the following:

With every new release, the query optimizer is enhanced. Oracle Database 11g Release 1 and Release 2 are no exception to the rule. Specifically, they introduce key improvements in the following areas: indexing, optimization techniques, object statistics and plan stability. The aim of this presentation is to review the new features from a practical point of view as well as to point out challenges related to them. In other words, to let you know what you can expect from the query optimizer when you upgrade to Oracle Database 11g.

Does CREATE INDEX Gather Global Statistics?

You can add the COMPUTE STATISTICS clause to the CREATE INDEX statement. It instructs the SQL statement to gather and store index statistics in the data dictionary, while creating the index. This is useful because the overhead associated with the gathering of statistics while executing this SQL statement is negligible. In Oracle9i, the gathering of [...]